Depth and inter-annual variation of crustacean diversity within Ecklonia maxima holdfast communities in the Benguela-Agulhas transition zone of the South African coast

Abstract Kelp holdfasts form complex, three-dimensional biogenic habitats that support diverse macroinvertebrate assemblages, yet the crustacean fauna within these structures which are an important contributors to secondary production and trophic transfer in kelp forest food webs remains poorly quantified. We characterised crustacean assemblages associated with holdfasts of the kelp Ecklonia maxima at Miller’s Point, False Bay, South Africa, to test for effects of depth and interannual variation. In 2024, we compared assemblages between shallow (2–5 m) and deep (6–8 m) holdfasts. To assess temporal change, holdfasts from shallow depths (2–5 m) were sampled annually from 2023 to 2025, with the 2024 shallow holdfasts serving as the shared shallow cohort for both comparisons across a total of 29 independently harvested holdfasts. Across all holdfasts, we recorded 10,560 crustaceans representing 131 morphospecies, 60 families and 10 orders, grouped into 12 functional categories based on mobility and feeding strategy. Deeper holdfasts hosted significantly more individuals and higher species richness (458.8 ± 70.7 SE individuals and 43.3 ± 4.2 SE species per holdfast) than in shallower holdfasts. Assemblage composition also varied significantly among years, with functional-group composition tracking these taxonomic shifts. These findings show that E. maxima holdfasts support unexpectedly high, spatially and temporally dynamic crustacean diversity, highlighting their role as biodiversity hotspots within kelp forests and pointing to the need for additional surveys to incorporate other taxa, kelp species and regions.